Although much is known about the control of both cardiac output and pulmonary ventilation, the mechanisms which precisely match both responses to the metabolic demands of exercise remain incompletely understood. These mechanisms are known to involve an interaction of feed-back and feed-forward (command signal) pathways. To obtain further insight into this interaction, this project is designed to study the degree of cardiopulmonary control coupling associated with the major mechanical changes present during normal exercise. In particular, the effect of the feed- forward drop in lung functional residual capacity (FRC) will be studied in terms of the cardiopulmonary reflexes involved in the awake dog. The basic question to be answered is whether this feed-forward lung volume pathway in combination with the known feed-back mechanisms can explain exercise responses. The role played by vagal afferents in mediating these responses will be determined by reversible cold block in awake dog.